This application is based on Patent Application No. 2000-236430 filed in Japan, the content of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an optical system comprising a plurality of prisms, and to a projection-type image display device provided with this optical system.
2. Description of the Related Art
There are projection-type image display devices which direct illumination light to a reflection-type display element, modulate the illumination light by a projection image displayed on the display element, and display the projection image on a screen by projecting the reflected modulation light through a projection optical system. Reflection-type liquid crystal displays (LCD), or digital micro mirror devices(trademark) (DMD(trademark), both DMD (trademark) and digital micro mirror devices(trademark) are trademarks of Texas Instruments) are used as the display element.
A reflection-type LCD modulates illumination light entering from an approximately perpendicular direction via a liquid crystal layer displaying the projection image, and reflects the image in an approximately perpendicular direction. A DMD(trademark) has micro mirror elements of variable directionality arranged in a plurality of rows on a plane. The direction of each mirror element is alternatively selectable from among two specified directions, which are selected in accordance with the displayed image. Illumination light comprises light of a displayed projection image reflected in one direction and modulated light of an undisplayed projection image reflected in another direction. The range of variance of direction of the mirror elements is very slight at the micro level, and the DMD(trademark) receives illumination light from a near-perpendicular direction, and reflects light in a second near-perpendicular direction.
In this way illumination light of modulated light from one direction must be directed to the display element in a projection-type image display device which modulates illumination light by a reflection-type display element. Accordingly, an optical system for directing illumination light to the display element without blocking the modulated light must be arranged medially to the display element and the projection optical system, and an optical system comprising a plurality of prisms is often used for this purpose.
An example of a conventional optical system is shown in FIG. 7. Parts (a) and (b) of FIG. 7 represent mutually intersecting cross section views of an optical system 50. The optical system 50 comprises two prisms 51 and 52. Prism 51 has three surfaces 51a, 51b, and 51c, and the surfaces 51a and 51b form an acute angle therebetween. Prism 52 also has three surfaces 52a, 52b, and 52c, and the surfaces 52a and 52b form an acute angle therebetween.
Prisms 51 and 52 are arranged such that surface 51b confronts surface 52b with an interval of a small distance therebetween. That is, a small air gap GP is formed between surface 51b and surface 52b. Surface 51b and surface 52b are mutually parallel, and the size (thickness) of the air gap GP is constant regardless of position.
The angle formed by the surfaces 51a and 51b of the prism 51 and the angle formed by the surfaces 52a and 52b of the prism 52 are equal to each other, and, accordingly, the surfaces 51a and 52a are parallel. An optical axis perpendicular to the surface 51a of prism 51 is referred to as optical axis Ax of the optical system 50. A display element is arranged perpendicularly to the optical axis Ax on the surface 51 a side of the optical system 50, and a projection optical system is arranged such that the optical axis of the projection optical system is parallel to the optical axis Ax on the surface 52a side of the optical system 50. Accordingly, the air gap GP, and the surfaces 51b and 52b, forming this air gap GP, are oblique to the optical axis of the projection optical system.
The direction of the optical axis Ax is referred to as the X direction, the direction perpendicular to the optical axis Ax within a plane perpendicular to the air gap GP is referred to as the Y direction, and the direction perpendicular to the optical axis Ax within a plane parallel to the air gap GP is referred to as the Z direction. Part (a) of FIG. 7 represents the cross section in the X-Y plane, and part (b) of FIG. 7 represents the cross section in the X-Z plane.
The light for illuminating the display element passes through the surface 51c of the prism 51 in the optical system 50. The light passing through the surface 51c [and entering] enters the prism 51 and reaches the surface 51b. The incidence angle of light on the surface 51b is set so as to exceed the critical angle, and the light is completely reflected by the surface 51b. The light completely reflected by the surface 51b reaches the surface 51a, is transmitted through the surface 51a, and impinges the display element approximately perpendicularly thereto.
Light impinging the display element is modulated and reflected by the projection image displayed on the display element. The modulated reflected light impinges the surface 51a, and passes through the prism 51, reaching the surface 51b. The entrance angle of this light on the surface 51b is less than the critical angle, and the light is transmitted through the surface 51b, crosses the air gap GP, and impinges the surface 52b of the prism 52. The light impinging the prism 52 reaches the surface 52a, is transmitted therethrough, impinges the projection optical system, is projected therefrom, and forms a projection image on the screen.
The modulated light is refracted when transmitted through the surfaces 51b and 52b. However, since the surfaces 51b and 52b are parallel, the optical path is also parallel both before passing through surfaces 51b and 52b and after passing through surfaces 51b and 52b. Since the air gap GP is oblique to the optical axis Ax, the size of the shift in the optical path before transmission through the air gap GP and after transmission through the air gap GP is different in the mutually perpendicular Y direction and Z direction. Therefore, although the light has the same point of origin, the origin point in the Y direction is positioned nearer to the projection optical system than the origin point in the Z direction. The shift of the Y direction and Z direction origin points in the optical axis Ax direction is referred to as the interval difference. The origin point in the Z direction is one point, however, the origin point in the Y direction is broadened.
Since the shift of these origin points causes distortion in the image formed by the projected light and reduces the quality of the displayed projection image, this shift must be suppressed as much as possible. For this reason, the size of the air gap is very small, approximately 10 xcexcm in a conventional optical system. FIG. 8 shows the relationship between the amount of defocus and the optical transfer function (OTF) when the size of the air gap GP is set to this degree. In FIG. 8, the curves marked by the symbols XY and XZ represent the OTF within the XY plane and the XZ plane, respectively. Both the amount of defocus of the horizontal axis or the OTF of the vertical axis is standardized when there is no air gap within the optical system. In FIG. 8, there is no great difference in the OTF of the XY plane and the OTF of the XZ plane, and excellent image forming performance is obtained.
When a color projection image is provided, light from a light source emitting white light is split into red (R), green (G), and blue (B) light, and each color light after splitting is modulated by separate reflection-type display elements. In this case an optical system having two air gaps of constant size are used, and a dichroic film is provided on one surface of each air gap, and, for example, red light is reflected and green light and blue light are transmitted by one dichroic film, and, for example, blue light is reflected and red light and green light are transmitted by the other dichroic film so as to split the red light, green light, and blue light.
Three display elements are provided to display the R component, the G component, and the B component of the projection image. The R light and the B light reflected by the dichroic film are completely reflected by different surfaces of the prism, and are directed to the corresponding display element. The light modulated and reflected by each display element follows the optical path in reverse and is combined within the optical system, and is projected by the projection optical system. In this case also, the size of the air gap is approximately 10 xcexcm, such that an excellent color image is displayed without color shift.
In recent years, extremely high intensity light has been used as illumination light in the optical system in accordance with demand for high luminance projection images. Although each prism of the optical system is manufactured using material of high transmittance, light energy is absorbed and a high temperature is reached, and swelling cannot be avoided. This swelling deforms the surfaces of the prism, and reduces the performance of the prism.
The deformation of the surface opposite the display element and the projection optical system is only somewhat connected to performance reduction. However, when the surfaces forming the air gap are deformed, both surfaces come into contact, and the parts in contact cannot produce complete reflection. When complete reflection cannot be attained, the illumination light cannot be directed to the display element and the optical system does not function.
In a projection-type image display device which displays a bright projection image, the air gap within the optical system is made large so as to prevent contact between the surfaces of the two prisms forming the air gap even when the prisms swell. FIG. 9 shows an optical system 50xe2x80x2 in which the air gap is large. Part (a) of FIG. 9 represents the cross section in the X-Y plane, and part (b) of FIG. 9 represents the cross section in the X-Z plane. In this optical system 50xe2x80x2, the air gap GP of the optical system 50 of FIG. 7 is enlarged from approximately 10 xcexcm to approximately 50 xcexcm. When the air gap becomes this large, the shift of the origin point of the light also increases, and the interval difference becomes extremely large.
FIG. 10 shows the relationship between defocus and OTF in the optical system 50xe2x80x2. There is a shift of approximately 0.05 mm in the maximum position of the OTF in the XY plane and the XZ plane, and the quality of the image displayed on the screen is greatly reduced.
Disadvantages accompanying an increase in the air gap in an optical system have long been known. However, simple and effective countermeasures have yet to be proposed. Although it is possible to suppress distortion by using an anamorphic projection optical system, an increase in cost is unavoidable. It becomes necessary to design the projection optical system in accordance with each individual optical system, and interchangeability is lost.
An object of the present invention is to provide an improved optical system and a projection-type image display device which is provided with this optical system and which is capable of providing a bright, high-quality projection image.
Another object of the present invention is to provide an optical system which is capable of suppressing the difference in the path of light in two directions, and a projection-type image display device which is provided with this optical system and which is capable of providing a bright, high-quality image.
These objects are attained by the present invention, in an optical system including a first prism having a first surface and a second surface forming an acute angle therebetween, and a second prism having a third surface and forming an air gap between the second surface and the third surface, wherein the cross section, perpendicular to the first surface and the second surface, of the air gap formed by the second surface and the third surface has a wedge shape which is wide at a part thereof near the first surface and narrower at a part thereof more distant from the first surface.